Radio tag for LFM radar

ABSTRACT

An RFID system using encoded digital information utilizing pulsed linear frequency modulation (LFM). The LFM waveform is sent from an aircraft or satellite and is received by a transponder. The LFM waveform is demodulated using both, an AM and an FM receiver. The demodulated data is compared to preprogrammed criteria tables, and after validation is decoded and utilized. Algorithms in the transponder are used to determine the frequency deviation and for calculating the direction of the slope of the LFM input signal. The valid RF signal is stored in a delay element, encoded with the transponder data using phase modulation (PM), and frequency modulation (FM). The tag transmission is synchronized to the input LFM waveform. The transmit/receive chopping signal prevents unwanted oscillations and is capable of randomization.

CROSS-REFERENCE TO RELATED APPLICATIONS

(Not Applicable)

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

(Not Applicable)

BACKGROUND OF THE INVENTION

A basic transponder receives a signal from a SAR (synthetic-apertureradar), and then retransmits the signal back to the SAR. The problemwith retransmitting the signal back to the SAR without any changes isthat the signal must compete with ground reflection noise from naturaland cultural ground clutter. If no type of modulation is used, then tostand out above the background noise, extra power and electronics areoften needed to increase the gain in the return signal, often requiringtwo antennas, one for receiving and one for transmitting. Mosttransponders do not provide any added information in the retransmittedsignal. In most cases the interrogator is a SAR located in an aircraftor satellite, and a transponder is located some distance away from theradar, normally on the ground.

The ability to provide command control and communications to and from atransponder has many potential applications. Besides the militaryapplications for battlefield management, intelligence gathering and thelike, there are commercial applications which include transponderstatus, other environmental status and emergency response. A RFID (radiofrequency identification) system contains two main elements, aninterrogator and one or more transponders. In the radar transmissionbetween the interrogator and transponder, a RF signal is encoded withinthe SAR pulse to provide information between the interrogator andtransponder, that is normally unknown to the other element.

U.S. Pat. No. 5,486,830 discloses a RFID system wherein digital codesare encoded in the SAR signal that is received by a transponder, alsocalled a RF tag or simply tag. A single antenna can be utilized by thistransponder to transmit and receive signals. This is accomplished by atime-gating method using a 50% duty cycle factor for setting the tag'stransmitting and receiving intervals, which are mutually exclusive. Thereceived SAR signal is mixed with a reference oscillator to provide adetected signal that can be decoded. Tag logic and timing circuitsmeasure the time between detected pulses and decode these pulses intodownlink commands from the SAR, symbols of the downlink commands areencoded in the spacing between the SAR pulses. Downlink commands containmode information that allows the SAR and tag to obtain a common pulseindex (coarse synchronization). However, in order to achieve finesynchronization the tag must average the measured time-of-arrival of anumber of pulses. After fine synchronization is achieved, and if socommanded, the tag will go into the uplink mode. The tag device phaseencodes its echo with a sequence containing both prescribed and periodicor pseudo-random patterns, containing status or information unknown tothe radar source's signal processor. A bi-phase (0/pi) modulator isutilized to allow selective amplification of +1 or −1 of the signalbefore retransmitting the signal back to the SAR. The signal processingat the SAR mixes a sequence identical to the prescribed periodic orpseudo-random selective amplification against the received echoes. Thisselective amplification spreads the spectrum of the natural or culturalechoes and de-spreads the tag's echo, making the retransmitted signalstand cut above the natural or cultural noise.

SUMMARY OF THE INVENTION

The present invention is an improvement over the other RFID systems.Prior art used simple pulses defined by their spacings in the SARpulses. The pulses represent symbols that encode the digital informationcontained in the SAR pulses.

The present invention uses SAR LFM (linear frequency modulation) pulsewaveforms. One advantage of using LFM pulse waveforms is that thesewaveforms are not noise sensitive. While the symbols of the encodeddigital code are determined by the time internal between the pulses asin U.S. Pat. No. 5,486,830, noise can cause an incorrect interval to bedetected and thereby generate an invalid message.

Two receivers, an AM and a FM receiver, are connected to an antenna andare used to demodulate their respective components of the LFM pulsewaveform. The output of the AM receiver is the demodulated envelope thatis proportional to the amplitude of the AM component of the LFM pulsewaveform. The amplitude and time duration are compared to apreprogrammed threshold and time duration criteria. The preprogrammedcriteria are stored in the tag DSP (Digital Signal Processor. If boththe threshold value and time duration are determined to be valid, thenthe demodulated FM portion of the LFM pulse waveform is checked forvalidity.

A reference local oscillator is needed to demodulate FM signal componentof a LFM pulse waveform. This is generated by delaying LFM pulsewaveform and mixing it with the non-delayed LFM pulse waveform. Aseparate reference oscillator is, therefore, not required. Thedemodulated waveform is then passed through a zero-crossing detector.The output of the zero-crossing detector is sampled and the samples arecounted by the DSP to estimate the average frequency over the pulseduration.

In order to determine the slope of the frequency deviation, a 90° powersplitter is added to the local oscillator, before the mixer. The powersplitter has two outputs, one in-phase and one in quadrature-phase withthe LFM pulse waveform. The signal component set of mixer, lowpassfilter, zero-crossing detector and sampler is replaced with twoidentical sets of components. The output of the set with the in-phasesignal is labeled “FM In-Phase”, and the output of the set with thequadrature-phase signal is labeled “FM quadrature-phase”. These twooutputs are then used by the DSP to determine the slope of the frequencydeviation.

The DSP utilizes the slope information and frequency deviation todetermine the message sent by the SAR. Though the use of PhaseModulation (PM), the tag can encode data to send back to the SAR.

By using RF switches, the signals can be directed so that the sameantenna can be used for receiving and transmitting. Since the sameantenna is used, a chop timing of the RF switches in required. There are3 stages in the chop signal: receive, transmit and blanking. Theblanking stage is required to prevent oscillations in the tag due toreflections from nearby objects. Some systems require randomizing theblanking time to prevent the radar from accidentally locking onto thespectral lines that are generated during the chopping of the RF signal.The blanking times can change pseudo-randomly each cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred embodiments of the presentinvention are shown in the accompanying drawings in which:

FIG. 1 is simplified block diagram of the tag;

FIG. 2 is the detailed block diagram of the tag;

FIG. 3 is the detailed block diagram of the digital signal processor;

FIG. 4 is the software flow diagram during message reception;

FIG. 5 is the signal flow through the AM receiver;

FIG. 6 is the signal flow through the FM receiver;

FIG. 7a is a graph of the low pass filter output of the FM receiver;

FIG. 7b is a table showing the frequency estimator code;

FIG. 8 is a timing diagram for synchronizing the tag and radar;

FIG. 9 is the signal flow during transmit;

FIG. 10 shows the chop timing of S1 and S2;

FIG. 11a is a diagram of the random chop generator;

FIG. 11b is a table showing the tag modulation states per pulse;

FIG. 12 shows the tag modulation.

FIG. 13a shows the FM receiver without slope direction;

FIG. 13b shows the FM receiver with slope direction capability;

FIG. 14a shows a graph of the I and Q outputs of the FM Receiver;

FIG. 14b is a table showing the slope measurement code;

FIG. 15 shows the slope polarity of a positive-going LFM; and

FIG. 16 shows the slope polarity of a negative-going LFM.

DETAILED DESCRIPTION

FIG. 1 shows the transponder device or tag 10 simplified block diagram.An antenna 1 receives the LFM (linear frequency modulation) inputwaveform from SAR. The received waveform is then fed into the AMreceiver 2, the FM receiver 3, and a storage element 6. The AM receiver2 demodulated output signal is then fed into the digital signalprocessor (DSP) 4. If the DSP 4 determines that the AM demodulatedoutput signal is valid, then the DSP 4 determines if the demodulatedoutput of the FM receiver 3 is valid. If both demodulated outputs arevalid, then the DSP 4 checks the validity of the received LFM inputwaveform against stored data. After the DSP 4 has determined that thedata encoded in the LFM input waveform is valid, and decoded themessage; the RF signal stored in the storage element 6 is modulated inthe modulator 5. The output of the modulator 5 is then fed back into theantenna 1 to be transmitted back to the originating source.

FIG. 2 shows a further breakdown of the components of the tag 10. Thetypical operating frequency is 8 to 12 GHz. The LFM input waveform issensed in antenna 1. A bandpass filter 11 is utilized to remove anyundesirable out of band signals out of the LFM input waveform. A firstdirectional coupler 12 passes the filtered LFM input waveform to a RFswitch 13 through the coupled side of the directional coupler 12. Ifswitch 13 is closed, then the received filtered RF signal will be fedinto a low noise amplifier 14. The RF switch 13 is only closed duringthe receive cycle. Switch 20 is open during the receive cycle. The RFfiltered LFM input waveform is then routed through the coupled side ofthe second coupler 15. A second low noise amplifier 22 amplifies thesignal, which is then fed into a power splitter 23. Up to the input ofthe power splitter 23, the LFM input waveform signal has beenidentically processed for both the AM receiver 2 and FM receiver 3.

The signal flow in the AM receiver 2 continues from the output of thepower splitter 23 though the bandpass filter 24. Bandpass filter 24removes unwanted out of band noise generated by the two previousamplifier stages 14 and 22. The output of filter 24 is applied to adiode detector 25. The detector 25 demodulates the envelope of the LFMinput waveform; the output of detector 25 is proportional to theamplitude of the LFM input waveform. A square law device is used in thedetector although other types of detector devices are possible, such asenvelope, synchronous, and log-video; however the square law detector isone of the most common forms. In the preferred embodiment, the detector25 consists of a tunnel diode and passive bias components and the designis not shown but is well known to practitioners of the art. The detectedoutput of the detector 25 is filtered by a low pass filter 26, whichremoves the higher order frequency terms which were generated during thenon-linear process of detection. A video amplifier 27 amplifies thefiltered output of filter 26. The output of the video amplifier 27 isapplied to the positive input terminal of a threshold comparator 28. Thevoltage at the negative input terminal of the comparator 28 is set by adigital to analog converter (DAC) 35. The output of the DAC 35 isdetermined by the digital word from the DSP 4. The output of thecomparator 28 is a digital logic “1” when the input at the positiveinput terminal is higher than the voltage at the negative inputterminal. The output of comparator 18 is a logic “0” when the input atthe positive input terminal is lower than the voltage at the negativeinput terminal. The output of the comparator 28 is labeled “AM AfterThreshold Detection” in FIG. 2 and is applied to a field programmablegate array (FPGA) 44 contained in DSP 4 shown in FIG. 3. The FPGA 44measures the pulse width of the comparator 28 output. This pulse widthis screened to be within the programmable minimum and maximum limits,which typically range between 10 μs to 150 μs. A pulse width, that doesnot pass the pulse width screening, will not enable the frequencyestimator within the DSP 4, as shown in FIG. 4, the tag software flowdiagram. An invalid pulse will cause no action in the frequencyestimator. Only after a pulse width has passed the pulse width screeningwill the frequency estimator be enabled.

The signal flow in the FM receiver 3 continues from the output of thepower splitter 23 though the low noise amplifier 29. The FM receivershares common parts with the AM receiver 2 from the antenna 1 to theinput of power splitter 23 as illustrated in FIGS. 2, 5 and 6. In the FMreceiver is a new input signal labeled “LO” (Local Oscillator) which isutilized as shown in FIG. 6. This signal is generated from the input LFMpulse waveform that shares common parts with the AM receiver and FMreceiver from the antenna 1 to the input of the second directionalcoupler 15, as illustrated in FIGS. 2, 5 and 6. At the coupler 15, thesignal passes through the main line of second directional coupler 15,and then through the delay line 16. The time delay, of the delay line16, in the preferred embodiment is 60 ns with the delay element being acoaxial cable. However, the design of the delay line 16 will operatewith any delay as long as the delay is constant over frequency. Otherdelay elements can be used such as SAW, BAW, optical fiber, tuned filterand digital circuits such as DRFM. These alternate delay elements mayrequire support circuits that are not shown here, but are well known inthe art. The output of delay line 16 then passes through the coupledside of the third directional coupler 17. The output of the thirddirectional coupler 17 then passes through the LO low noise bufferamplifier 34. The output of the buffer amplifier 34 is then fed into theLO input of the mixer 30.

The LO signal at the mixer 30 input is a delayed replica of the LFMinput waveform received at the antenna 1. This LO signal and the mixer30 comprise the frequency detector in the FM receiver 3 of the originalLFM pulse waveform. The frequency demodulation is accomplished bymultiplying the delayed signal (LO) with the non-delayed signal in themixer 30. The formula for the demodulation output of the low pass filter31 after the mixer 30, for an LFM input, is given below:

Frequency output (Hz)=delay (seconds)×LFM (Hz/second).

The following is an example calculation of the above formula. The taghas a time delay of 60 ns, and an input LFM waveform is applied at 8.5GHz with a positive-slope ramp deviation of 100 MHz and a pulse width of27.7 μs. The frequency output is 60e-9×100e6/27.7e-6 or approximately216 KHz. FIG. 7 shows the output of the low pass filter 31 for thisinput signal.

After the low pass filter 31, the FM signal is limited with azero-crossing comparator 32 that converts the signal into a one bitdigital value and the sampling of the one bit digital value iscontrolled by the Sample Clock input, from the FPGA 44 of the DSP 4 asshown in FIG. 3, to the sampler 33. Sampler 33 may be a clocked registeror an other circuit that can hold the value of the digital bit until thenext sample clock is received by the sampler 33. The output of sampler33 is labeled “LFM After Sampling”. The output of sampler 33 is appliedto the microprocessor 42 in the DSP 4 in FIG. 3, for storage in thememory RAM 41 for further processing by the microprocessor 42. It isonly processed if the output of comparator 28 is within thepreprogrammed valid range of widths.

FIG. 4 is a diagram of the processing steps of the microprocessor. Theoutput of the FM receiver 3, which is the output of the sampler 33, isfed into the microprocessor 42 for frequency estimating. The frequencyestimator counts the zero-crossings of the demodulated FM signal fromthe output of sampler 33, and calculates the average frequency duringthe valid receive interval. The code for this frequency estimator isprovided in FIG. 7B. In FIG. 7B, the variable “SS” represents a validreceive interval. If the variable “SS” is not equal to 1, then theoutput of the threshold comparator 28 is not a digital logic 1. Allreceived signals that do not generate a digital logic 1 at the output ofcomparator 28 are considered to be noise and are not processed further.The variable “SLIN” is the digital bit in the output signal orcomparator 33 labeled as “FM After Sampling”. The variable “clock” is a2 MHz sample clock. The actual frequency can be changed but the programmust make the variable “clock” equal to the actual sample clock rateused. The variable “data” is the sample length. The sample lengthdynamically changes with the width of the input pulse.

In the previous example, the sample length would be 2 Mhz×27.7 μs, orapproximately 55 samples. Using the frequency estimator of FIG. 7B andthe 100 MHz deviation from the example, the calculated frequency outputis 219 KHz. This is within 2% of the theoretical value of 216 KHz and issatisfactory for the RFID system. The FM processing continues in FIG. 4,where the measured frequency is compared to a table of validfrequencies. If the comparison is true, then the tag builds a messagefrom the data that was encoded in the pulse width and frequencydeviations of the received input LFM pulse waveform. Although thisimplementation is described in software, faster operation could beobtained using a matched filter in the DSP section.

The tag 10 checks the validity of the data in the message by examiningthe data fields and performing a checksum. An example of a checksum isthe CCITT 16 bit CRC (cyclic redundancy check). However, other types ofchecksum may be used since the type of checking does not materiallyaffect the performance of the tag. The tag's 10 checksum is comparedwith the transmitted checksum and if they match then the message isvalid and the tag declares Message Complete. A valid message willtypically contain the radar pulse width, radar PRI, and transmittedchecksum, and may include other information for the tag 10 or about theradar depending on the system operation.

A valid message will enable data transmission from the tag 10. The tag10 must align its transmission so that its pulse is on top of, orsynchronous with, the radar pulse. The tag 10 does this by loading theinitial radar pulse width and radar PRI (pulse repetition interval) in acounter of the DSP 4, but not starting the counter. These values in thecounter are the pulse width and PRI from the valid message that wereobtained after the tag declares Message Complete. At the starting edgeof the initial radar pulse, a digital control signal, download_intn, isgenerated, which is a request by the FPGA 44 to the microprocessor 42for modulation data. This data is used in the validating of any decodedvalid AM and FM demodulated output to the DSP 4. From the data decodedfrom the initial SAR pulse, the tag 10 estimates when the next pulsewill be received. The exact timing for the ld_symbol during the initialpulse is not critical, however the ld_symbol must occur before the firsttransmit pulse. The download_intn is a request from the FPGA 44 to themicroprocessor 42 for new modulation data. This data is used in the nextpulse one PRI later. Ld_symbol latches the modulation data into the FPGA44 before rf_detect occurs. The tag 10 waits for the leading edge of theinitial radar pulse and when it detects the leading edge it transmitsafter the initial PRI. During the following pulses the tag loads thecounter with the actual radar PRI and so transmits this PRI repeatedly.The PRI timer gets resynchronized to each incoming RF to take intoaccount inaccuracies in the FPGA clock. After the initial PRI, an uplinkgate signal is generated to control the waveform XMT, which is theenvelope of the RF transmission from the tag. In this way the tag 10becomes synchronized with future pulses from the radar. The errorbetween the tag pulse and radar pulse is typically less than ±100 ns.The process of synchronizing the tag with the radar is illustrated inFIG. 8. The following is a description of the signals named in thefigure:

“rf” is the envelope of the received RF signal in the LFM pulsewaveform;

“latency” is the time delay from the received RF to the digital outputof the threshold detector 28, labeled as “AM After Threshold Detector”,and occurs due to circuit delay in the AM receiver 2;

“init_pri” is the initial PRI transmission from the tag 10;

“gate” is the width of the received LFM pulse waveform;

“pri” is the PRI of the received LFM pulse waveform, and the PRI of alltag transmissions except for the initial PRI;

“rf_detect” is the same as the output of the threshold detector 28,labeled as “AM After Threshold Detector”;

“uplink gate” is the pulse width of the tag transmission;

“download_intn” is a digital word signal sent by the FPGA 44 to themicroprocessor 42 and is a request to the microprocessor 42 formodulation data;

“ld_symbol” initializes the FPGA 44 with new modulation information fromthe microprocessor 42;

“xmt” is the envelope of the RF transmission from the tag 10;

“chopping signal” refers to a toggling state of the controlling signalsS1 and S2 that control RF switch S1 13 and RF switch S2 20 respectively

The LFM pulse waveform signal flow during transmit is shown in FIG. 9.The dashed line was previously described n the received signal flow andis also part of the transmission. The LFM input pulse waveform uses thesame components as the AM 2 and FM 3 receivers up to the input of thesecond directional coupler 15. The LFM pulse waveform signal passesthrough the main line of second directional coupler 15, followed by thedelay line 16. It then passes through the main line of third directionalcoupler 17. The LFM pulse waveform signal that was stored in delay line16, after passing through the directional coupler 17 is electricallycoupled to the variable attenuator 18, phase modulator 19, RF switch S220 (closed in transmit), power amplifier 21, back through the main lineof the first directional coupler 12 and through bandpass filter 11 andtransmitted from the antenna 1.

To allow the tag to share the same antenna 1 for both receive andtransmit, the chopping signals S1 and S2 use the same frequency and thelength of delay for the delay line 16 is based on the frequency used inthe timing of chopping signals S1 and S2. The timing of S1 and S2 isshown in FIG. 10 for a 60 ns delay line 16. In this figure the receivetime is 60 ns, transmit time is 60 ns, and the blanking time (when thetag is neither transmitting nor receiving) is 60 ns. The transition timefrom on to off and vice versa due to the RF switches S1 13 and S2 20 is10 ns. A complete cycle in the example in FIG. 10 is 180 ns. Theblanking interval in FIG. 10 prevents oscillations due to reflectionsfrom nearby objects.

Some systems require randomizing the blanking time to prevent the radarfrom accidentally locking onto the spectral lines that are generatedduring the chopping of the RF signal. The randomizing circuit for choptiming is shown in FIG. 11a. The blanking time changes pseudo-randomlyeach cycle from 60 ns to 200 ns with a resolution of 20 ns. The designis a [7,1] maximal length generator and the initial seed is binary1111111. The sequence repeats every 127 chop cycles. The circuits forthe chop timing and chop randomization are contained in the FPGA 44.Other randomizing circuit designs could also be utilized.

The pulse is modulated with tag data using the signal phase modulator 19as shown in FIG. 2, by the signal labeled “PM”. The phase modulator 19is a 5-bit phase shifter and is capable of placing frequency modulationand phase modulation on the RF signal. A typical modulation consists ofa linear frequency slope and a bi-phase value. During the process ofmodulation, the binary data in the message is encoded. An example of anencoding scheme is shown in FIG. 11b Three bits of data from the tag areplaced on each pulse using different states of frequency deviation andphase. However, the tag is capable of generating other modulationvalues, and the design is capable of a broad range of frequency andphase modulation.

The RF output from the tag 10 for the example in FIG. 11b is shown inFIG. 12. It shows that the tag adds chop modulation, phase modulation,and linear frequency modulation (LFM) to the radar pulse. The transmittime of the tag is synchronous with the radar pulse. The plus symbol inthe LFM+ indicates that the tag has modified the LFM slope of theoriginal radar LFM signal.

The tag adjusts the gain in the transmit path using the AM signal at thevariable RF attenuator 18 in FIG. 2, the signal being generated from themicroprocessor 42 in FIG. 3. The AM signal controls a 5-bit digitallytunable RF attenuator. However, the signal may also be an analog voltagein another implementation and the type of RF attenuator does notmaterially affect the tag 10. The microprocessor 42 sets the amplitudevalue of the AM. The gain is typically set to 63 dB (peak) as measuredfrom the antenna RF input to the antenna RF output.

FIG. 13a is a partial block diagram of the FM receiver 3 in FIG. 2.These components can not distinguish between increasing and decreasingLFM signals, because they respond equally to positive and negativeslopes from the FM signal. The preferred embodiment uses a mixer 30,followed by a low pass filter 31, a zero-crossing detector 32 and asampler 33. The output of the sampler 33 is labeled LFM After Sampling.This output is then processed by the microprocessor 42 in the DSP 4.

This restriction is overcome by quadrature detection and processing ofthe I and Q signals as shown in FIG. 13b of FIG. 13. FIG. 13b shows theblock diagram of the revised design for calculating the slope. Theamplifiers 29 and 34 are unchanged from the preferred embodiment. A 90°power splitter 100 is now placed between the output of amplifier 34 andmixer 30. The 90° power splitter 100 produces two outputs, one isin-phase with the input signal and the other output is out of phase by90° and is in quadrature-phase with the input. Each output is processedby a separate but identical set of components. The outputs of the twosets of components are labeled “FM In-Phase” (I) and “FMQuadrature-Phase” (Q). The output from the revised FM receiver 3 labeled“FM in-phase” uses the component set: mixer 30I, lowpass filter 31I,zero-crossing detector 32I and sampler 33I. The output from the revisedFM receiver 3 labeled “FM Quadrature-Phase” uses the component set:mixer 30Q, lowpass filter 31Q, zero-crossing detector 32Q and sampler33Q.

These two output signals are further processed by the microprocessor 42in the DSP4, to find the polarity of the slope. The analog outputs of Iand Q prior to digitizing are shown in FIG. 14a, using the same inputexample as previously discussed.

The processing steps for the slope polarity are shown in FIG. 14b.SLOPE[n] is the output and is initialized to zero. The result is +1 whenthe slope is positive and −1 when the slope is negative. The dataprocessing is accomplished by the microprocessor 42 inside the DSP 4.The output of the slope polarity is shown in FIG. 15 for the positiveslope of the example. The top trace is the detection output of the AMreceiver 2 threshold comparator 28 and is shown for reference only, andthe bottom trace is the slope polarity scaled by ½. FIG. 16 is theoutput for a negative slope.

The algorithm in FIG. 14b provides multiple outputs, separated in timebut having identical value, and any one of the multiple values issuitable for defining polarity. Typically the microprocessor 42 willselect the first value. The number of multiple values is a function ofthe input FM deviation and pulse width.

The above teachings are illustrations of preferred embodiment of thepresent invention. It should be noted that modifications to theinvention, such as would occur to those of ordinary skill in the art,are also within the intended scope of the present invention.

What is claimed is:
 1. A tag apparatus, capable of detecting, decodingand validating a radar message contained in an AM and FM signalcomponent of a received LFM (linear frequency modulation) pulse waveformfrom a radar source, and encoding and retransmitting, back to the radarsource, a tag message contained in a PM and FM signal component of amodified LFM pulse waveform, the apparatus comprising: a) an antennainput circuit for receiving the LFM pulse waveform having digitalinformation; b) an AM/FM signal component validation circuit, inelectrical communication with said antenna, for detecting, decoding andvalidating said AM and FM components in the LFM waveform, and comparingthe components to a predetermined criteria, in electrical communicationwith said antenna; c) a delay element in electrical communication withthe antenna for storing the received LFM pulse waveform; d) a choppedrepeater circuit, in electrical connection with the delay element forencoding the stored LFM pulse waveform in the delay element with apreprogrammed AM and a FM predetermined criteria to form a modified LFMpulse waveform; e) timing synchronization circuitry for synchronizingthe chopped repeater circuit of the modified LFM pulse waveform withreception of the received LFM pulse; and f) an antenna output circuit,in electrical communication with the chopped repeater circuit,retransmitting of the modified LFM pulse back to the radar source. 2.The apparatus as recited in claim 1, in which the AM signal componentvalidation circuit further comprises a threshold comparator circuit toproduce an AM After Threshold Detect signal when the amplitudedeviations of a demodulated AM component envelope of the LFM pulsewaveform, exceed a DAC(Digital-to-Analog Converter) threshold voltage,said DAC threshold voltage being produced from a preprogrammed DACthreshold voltage stored digitally in a DSP in the tag.
 3. The apparatusas recited in claim 2, in which said AM signal validation circuitfurther comprises a time duration circuit, said time duration circuitvalidating the amplitude deviation time duration of the AM AfterThreshold Detect signal by comparing the AM After Threshold Detect timeduration against a preprogrammed time duration criteria table containedwithin the DSP in the tag.
 4. The apparatus as recited in claim 3,wherein said FM signal component validation circuit further comprises aFM demodulation circuit, said demodulation circuit including a mixer anda local oscillator, said local oscillator frequency being in electricalcommunication with the mixer and operative to generate a localoscillator frequency signal by passing the LFM pulse waveform through afixed constant time delay element.
 5. The apparatus as recited in claim4, in which said FM signal validation circuit further comprises afrequency estimator circuit, said frequency estimator circuit beingenabled when the AM After Threshold signal time duration has beenvalidated by information in the preprogrammed time duration criteriatable.
 6. The apparatus as recited in claim 5, in which said FM signalvalidation circuit further comprises a frequency estimator circuit, saidfrequency estimator circuit including a zero-crossing circuit, saidzero-crossing circuit being operative to convert the FM signal componentinto a one bit value, said one bit value being sampled by a clock andapplied to the DSP, the DSP calculating an average frequency during theAM After Threshold time duration of the AM signal component of the LFMpulse waveform.
 7. The apparatus as recited in claim 6, wherein said FMvalidation circuit further comprises a preprogrammed digital codes tablefor decoding a message contained in the received LFM pulse waveform. 8.The apparatus as recited in claim 7, wherein said AM/FM signal componentvalidation circuit further comprises a slope polarity detector fordetermining the polarity of the slope, and comparing the slope to aslope criteria preprogrammed table for validation.
 9. The apparatus asrecited in claim 8, wherein said slope polarity detector includes a 90°power splitter in electrical communication with the delay element andits outputs, one at 0° and the other at 90° relative to the firstoutput, in electrical communication with at least one frequencyestimator circuit, the frequency estimator circuit outputs are inputsinto the DSP(Digital Signal Processor), the DSP using a software programin determining the polarity of the slope, and comparing the slope to aslope criteria preprogrammed table for validation.
 10. The apparatus asrecited in claim 9, wherein the chopped repeater circuit furthercomprises a RF attenuator for the amplitude of the modified LFM waveformand a phase modulator for modifying the amplitude, phase modulation andfrequency modulation of the stored LFM waveform and thereby generatingthe modified LFM waveform.
 11. The apparatus as recited in claim 10,wherein the phase modulator further comprises a phase shifter that iscontrollable up to a 5-bit resolution, that is in electricalcommunication with the delay element for receiving the stored LFMwaveform and generating the modified LFM waveform.
 12. The apparatus asrecited in claim 11, wherein the phase shifter further comprises anattenuator that is controllable up to a 5-bit resolution.
 13. Theapparatus as recited in claim 1, wherein said timing synchronizationcircuitry is connected to the DSP, and is operative to generate achopping signal that is preprogrammed.
 14. The apparatus as recited inclaim 13 wherein the timing synchronization circuit is controlled by theDSP for generating a random or pseudo-random modified LFM waveformtransmission from a predetermined criteria table.
 15. The apparatus asrecited in claim 14, wherein the chopping signal comprises a blankingsignal portion, said blanking signal portion is generated when the tagapparatus is neither receiving nor transmitting.
 16. The apparatus asrecited in claim 1, further comprising a plurality of RF switches, saidplurality of RF switches being operative to receive the LFM pulsewaveform and the modified LFM waveform and to selectively direct onlyone of the received LFM pulse waveform and the retransmitted modifiedLFM pulse waveform to the antenna.
 17. The apparatus as recited in claim16, in which the chopped repeater circuit is operative to synchronizethe transmission of the modified LFM waveform to a front edge of areceived pulse of the LFM pulse waveform from the radar source.
 18. Theapparatus as recited in claim 17, wherein the delay element is selectedfrom a group comprising a coax cable, SAW, BAW, optical fiber, tunedfilters and DRFM digital circuits.
 19. The apparatus as recited in claim18, wherein RF switches are operative to select between the input andoutput circuits of the antenna, thereby permitting the use of a singleantenna.
 20. A method of decoding and encoding digital informationcontained within amplitude and frequency modulated components of areceived LFM pulse waveforms from a radar source, the method comprisingthe steps of: a) receiving the LFM pulse waveform; b) detecting theamplitude deviation of the LFM pulse waveform using an AM receiver; c)detecting frequency deviations in the LFM pulse waveform using an FMreceiver; d) determining if the received demodulated amplitude andfrequency deviation components of the LFM pulse waveform satisfies apredetermined criteria; e) comparing preprogrammed time durationcriteria data to the deviation components of the LFM pulse waveform; f)decoding the digital information in the received LFM waveforms havingcomponents that satisfy the time duration criteria; g) communication ofthe decoded information to a tag; h) generating tag transmit data inresponse to the decoded information; i) storing the received RF signalfor possible retransmission; j)encoding tag transmit data in the storedRF signal using PM and FM, the deviation components being stored data ina preprogrammed criteria table; k) chopping the RF signal duringtransmission; and l) transmitting the modified LFM pulse waveform insync with the reception of an LFM pulse waveform with the encoded tagdata.
 21. The method of claim 20, further comprising the step ofdetecting the phase slope of the frequency deviations in the LFM pulsewaveform.
 22. The method of claim 21, further comprising the step ofrandomizing the length of the time duration of the chopped RF signalduring tag transmission.
 23. The method of claim 22, further comprisingthe step of adding a blanking signal time within the time duration ofthe chopped RF signal.
 24. The method of claim 23, further comprisingthe step of synchronizing the transmission of the modified LFM pulsewaveform with the leading edge of a front edge of a received pulse ofthe LFM pulse waveform from the radar source.
 25. The method of claim24, further comprising the step of selectively enabling a single antennato either receive the LFM pulse waveform and transmit the modified LFMpulse waveform.
 26. The method of claim 21, further comprising the stepof pseudo-randomizing the time duration of the chopped RF signal duringtag transmission.